Estimation for engraving with the 3rd axis is being optimised. It may be inaccurate at the moment.
In case the estimation is noticeably off compared to your actual engraving time, e.g. by over 10% or a minute, I would appreciate it if you could email the original image to yiyu.inbox@gmail.com for analysis.
Estimations for dithered images are known to be off. I don't have a solution for this. ¯\_( ͠❛ ͜ʖ͠❛ )_/¯
I will add support for Gray mode after LP finishes upgrading the Gray mode in both the app and the firmware.